1. Field of the Invention
The present invention relates to a motor control device for compensating backlash between a movable part to be driven by a motor and a driven part to be driven by the movable part.
2. Description of the Related Art
A servomotor is coupled to a feed axis of an industrial machine or a machine tool, or for example, to an arm of an industrial robot (mechanical movable part). The rotation of the servomotor is converted into linear motion of, for example, a table via, for example, a ball screw, or the conduction velocity of the servomotor is reduced by a reduction drive.
In the ball screw or the reduction drive, there may be a difference between a stop position in a positive direction toward a given position and a stop position in a negative direction. Such a difference is generally referred to as “backlash”, and could impair the positioning accuracy.
FIGS. 3A to 3C are drawings to explain backlash. FIG. 3A illustrates a movable part WA to be moved by a motor (not shown) and a driven part WB to be driven by the movable part WA. The movable part WA has, at its both ends, projecting portions A1 and A2. The driven part WB has, at its center, a protruding portion B. Thus, when, for example, the movable part WA moves rightward, an inner end of the projecting portion Al of the movable part WA engages with one end of the protruding portion B of the driven part WB. This moves the movable part WA and the driven part WB together to the right-hand side.
When the motor is inverted, the velocity of the movable part WA is inverted so that the moving direction of the movable part WA is changed from rightward to leftward (FIG. 3B). Then, when an inner end of the projecting portion A2 of the movable part WA engages with the other end of the protruding portion B of the driven part WB as shown in FIG. 3C, the movable part WA and the driven part WB move together to the left-hand side.
As described above, the movable part WA has to move by a predetermined moving amount referred to as backlash when being inverted, before the movable part WA engages with the driven part WB. FIGS. 3A and 3C show backlash having a length L. The backlash may reduce the positioning accuracy.
In order to prevent this reduction, it has been known to calculate a compensation amount for backlash, and to add the compensation amount to a position command for a motor at the time of inverting. In Japanese Unexamined Patent Publication (Kokai) No. 2014-054001, a compensation amount for backlash is calculated based on a position error between a movable part and a driven part at present, and a position error between the movable part and the driven part when the movable part engages with the driven part.